Circularly polarized slot antenna



Feb. 14, 1961 E. J. WILKINSON 2,972,147 CIRCULARLY POLARIZED SLOT ANTENNA Filed July 9, 1958 FIG. 2

INVENTOR.

ERNEST J. WILKINSON ATTORNEY United States Patent 6 CIRCULARLY POLARIZED SLOT ANTENNA Ernest J. Wilkinson, Newtonville, Mass., assiguor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed July 9, 1958, Ser. No. 747,540

7 Claims. (Cl. 343-767) This invention relates to antennas and more particularly to antennas employing a slot radiator and especially adapted for use on aircraft, such as high speed planes and missiles.

Due to the high speed of modern aircraft and missiles it is important that the size of protuberances from the surfaces of the craft be kept small, or if possible eliminated. Since most such craft have communication equipment of one kind or another mounted thereon, with the attendant requirement for antennas; considerable work has been done toward reduction of the size of such antennas and their location within the body of the aircraft or mounting flush with surface of the craft. Antennas employing a slot radiator are particularly useful in the latter connection, the aperture being located flush with the skin of the craft and preferably backed up by a resonant cavity for exciting the slot. A slot radiator, however, produces a linearly polarized radiation pattern which is unsatisfactory in certain applications. That is, some systems require an antenna having the characteristics of a slot radiator (electrical as well as mechanical), but circularly rather than linearly polarized patterns. I-Ieretofore, antennas having this capability have been unavailable.

Accordingly, it is a principal object of the present invention to provide an antenna capable of producing circularly polarized slot radiator patterns; that is, patterns relatively broad in one principal plane and relatively narrow in the other principal plane.

Another object of the present invention is to provide a high frequency antenna having the above electrical characteristics but having little or no aerodynamic drag.

Still another object of the invention is to provide an aircraft antenna having the above electrical characteristics and yet be capable of being entirely contained within the skin of the craft while consuming a minimum of space within the craft.

These and other objects of the invention are realized by the combination of a slot antenna backed up by a cavity, and excited by a dipole radiator positioned in the plane of the slot. The dipole is tilted at a small angle to the long axis of the slot thereby producing a component of dipole field in a direction perpendicular to the long axis of the slot which, in turn, excites the slot. The resultant aperture field of the slot when tuned by a cavity behind the slot can be adjusted such that when re-radiation from the slot is combined with direct radiation from the dipole the resultant radiation field is circularly polarized.

While intended primarily for use on aircraft, it will be evident from the following description that the antenna is equally applicable for use on other vehicles and in fixed installations such as in the wall of a building or tower.

For a better understanding of the invention and other objects and features thereof, reference is had to the following description of a preferred embodiment taken with variations in the angle of tilt of the dipole.

in connection with the accompanying drawings, in which:

Fig. 1 is a perspective view of the antenna of the invention; and

Fig. 2 is a perspective of the dipole of the antenna of Fig. l, enlarged to illustrate its structural details.

Referring now to Fig. 1, there is shown a conductive plate 10, which may be the skin of the aircraft, having formed therein a rectangular slot 12 having side edges 14'and end edges 16. It is known in the art to utilize as an antenna or radiating structure, a conductive plate having a rectangular slot of this configuration formed therein. A rectangular cavity 18 is formed back of aperture 12 by a conductive wave guide structure closed at the rear by plate 20. The aperture 12 would normally be closed by a thin window of radiation permeable material to prevent entry of rain, snow or dust, and to minimize drag, but since such closure is not a feature of the present invention it has been omitted from the drawing. Dimsensions of the antenna thus far described which have been successful are indicated in the drawing and are tabulated as follows:

Slot width, approximately wavelength Slot length, approximately wavelength Cavity depth, approximately wavelength The wavelength referred to is that of the midfrequency of the desired frequency band, the dimension applying to an air filled cavity.

A dipole 22, fed by a balun 24 lying along an axis normal to the plane of the slot, is positioned in the aperture of the slot with the radiating element of the dipole tilted slightly with respect to the long axis of the slot. The dipole is of known configuration, including a coaxial line projecting through the back wall 20 of cavity 18, the outer conductor split to divide it into two semi-circular portions at the outer end. The inner conductor and one of the radiating elements are conductively secured to one portion, and the other radiating element is secured to the other portion. As is well known in the art, this construction permits a balanced load (dipole) to be fed from an unbalanced coaxial line input.

As was mentioned earlier, the dipole is tilted with respect to the long axis of slot 12, a tilt of 18 having been found satisfactory with the indicated slot and cavity dimensions. This tilt produces a component of dipole field in a direction perpendicular to the long axis of i the slot, this excitation field being approximately in phase quadrature with the current on the dipole over a distance comparable to the width of the slot, which is small in terms of wavelength; namely, of the order of /a wavelength. The phase and amplitude of the resultant field at the aperture of the slot 12 relative to the current on the dipole 22 can be adjusted by changing the depth of cavity 18. The vertically polarized reradiation produced by this resultant aperture field of the slot when added to the relatively smaller vertically polarizer radiation produced directly by the dipole produces a resultant vertically polarized radiation field which is exactly out of phase with the horizontally polarized radiation field of the dipole, provided the depth of cavity 18 is of the proper dimension. A nominal depth of M4 (free space A) provides the proper phase relationship, but this is subject to some minor adjustment The angle of tilt of the dipole mainly affects the amplitude of the two fields, so long as the tilt angle is small, and by proper adjustment, the amplitudes of the resultant vertically polarized field and the horizontally polarized field can be made substantially equal, this condition coupled with the 90 phase relationship between the Patented Feb. 14, 1961,

horizontally and vertically polarized fields providing circular polarization. It has been found that almost perfect circular polarization can be achieved at a given frequency, and that good circularity (less than 3db) can be maintained over at least a 20 percent bandwidth.

The radiation pattern of the antenna corresponds approximately to that which would be produced by a linearly polarized slot having the same dimensions, an average beam width of approximately 120 in the direction of the narrow dimension of the slot and a beam width of approximately 60 in the direction of the long dimension of the slot. It has been found that the polarization ellipse does not change with aspect angle, the patterns being substantially identical for all components'in the plane of the slot and differing only slightly in the plane perpendicular to the slot where the patterns are very broad and affected more by the finite ground plane, that is, plate 10, in which the slot is formed.

While a specific embodiment of the invention has been described, including dimensions of a combination which has been found to operate satisfactorily, these dimensions are subject to some variation while retaining the intended operating characteristics. The essential attributes of the combination are substantially equal amplitude resultant vertically polarized and horizontally polarized fields in phase quadrature, and these may be obtained with a variety of combinations of dimensions. In general, the width of slot 18 is below that required for cut-off, i.e., less than M2, and preferably about M3, and the length must be greater than M2. As previously indicated, a length of 0.9x for a slot width of M3 has been found satisfactory. For other slot dimensions the cavity depth may have to be varied slightly from the indicated M4 dimension, as well as the angle of tilt from the indicated angle of 18 as measured from the long axis of the slot. It is to be understood, also, that the dipole need not be of the precise construction shown and described, but may be of any of the many forms known to the art. For example, it has been found that the antenna can be matched to a standard 50 ohm line much better when the dipole is folded. It will be appreciated, too, that the antenna of the invention is usable for either transmitting or receiving service depending upon the type of radio frequency transducer connected to the remote end of the dipole feed line.

From the foregoing it is seen that applicant has provided an antenna in which the same element, namely, the dipole, is utilized to excite a linearly polarized slot radiator and also to radiate directly. The result is a circularly polarized antenna capable of producing pattern shapes heretofore possible only with a linearly polarized antenna.

What is claimed is:

1. An antenna comprising, in combination, a conductive plate having a slot therein, means arranged in said slot for producing a first radiation field principally polarized in one direction and to excite said slot, and a cavity arranged behind said slot and operative to produce a second radiation field at the aperture of said slot polarized orthogonally to said first field and in phase quadrature therewith.

2. A circularly polarized antenna system including a conductive plate having a rectangular aperture therein, a conductive structure arranged behind said aperture and connected to said plate to define a cavity open at said aperture, and a dipole element disposed in the plane of said aperture and tilted relative to the longer axis of said aperture.

3. A circularly polarized antenna system including a conductive plate having a rectangular aperture having length and width dimensions formed therein, a hollow conductive cavity structure connected to said plate behind said aperture and open at said aperture, and a dipole arranged with its radiating elements disposed in the plane of said aperture centrally thereof with the axis through the radiating elements tilted at a small angle relative to the length dimension of said aperture.

4. A high frequency broad band circularly polarized antenna system comprising a conductive plate having a rectangular aperture therein, said aperture having length and width dimensions of substantially nine-tenths and one-third of the mid-wavelength of the desired pass band, respectively, a conductive structure arranged behind said aperture, and connected to said plate to define a cavity open at said aperture, the depth of said cavity being substantially one-quarter of the mid-wavelength of the desired pass band, and a dipole arranged with its radiating elements in the plane of said aperture and supported on a transmission line extending centrally through said cavity and lying on an axis normal to the plane of said aperture, the axis of said radiating elements being tilted substantially eighteen degrees with respect to the long axis of said aperture.

5. A high frequency broad band circularly polarized antenna system having a broad radiation pattern in one principal plane and a relatively narrow radiation pattern in the other principal plane comprising, a conductive plate having a rectangular slot therein having length and width dimensions of substantially nine-tenths and onethird of the mid-wavelength of the desired pass band, respectively, oriented with its width dimension parallel to said one principal plane, a rectangular conductive structure arranged behind said slot and connected to said plate to define a cavity open at said slot, the depth of said cavity from the plane of said slot being substantially one-quarter of the mid-wavelength of the desired pass band, and a dipole arranged with its radiating elements in the plane of said slot and supported on a coaxial transmission line extending centrally through said cavity and lying on an axis normal to the plane of said plate,

the axis of said radiating elements being tilted approximately 18 with respect to the length axis of said slot.

6. A broad band circularly polarized antenna having a radiation pattern corresponding substantially to that of a slot radiator, said antenna comprising a conductive sheet having a rectangular aperture therein, a cavity arranged behind said aperture, and a dipole arranged in the plane of said aperture and tilted with respect to the long axis of said aperture.

7. A broadband circularly polarized antenna having a radiation pattern corresponding substantially to that of a slot radiator, said antenna comprising a conductive sheet having an elongated aperture therein, a cavity arranged behind said aperture, and a dipole arranged in the plane of said aperture and tilted with respect to the long Kolar et al. May 28, 1957 Chlavin Aug. 5, 1958 

